The present invention relates to a radio communication apparatus capable of transmission and reception at a time.
A general configuration of a radio communication apparatus capable of transmission and reception at a time will be explained with reference to FIG. 1. A local signal having a local oscillation frequency f.sub.L corresponding to a transmission frequency f.sub.T and a reception frequency f.sub.R is generated at a frequency synthesizer 17. The frequency of the local signal can be changed by control data supplied through a bus 101 to the frequency synthesizer from a controller (not shown). The local signal is applied to a frequency modulator 18 and a reception mixer 21. In the frequency modulator 18, a signal modulated by a modulation signal applied thereto from a terminal 16 thereof and the local signal are added to or subtracted from each other at a transmitting mixer, thus generating a transmission signal of a transmission frequency f.sub.T. This signal, after amplified by a power amplifier 19, is sent to an antenna (not shown) from a terminal 23 through an antenna duplexer 20. The reception signal of frequency f.sub.R induced in the antenna, on the other hand, is supplied from the terminal 23 to the antenna duplexer 20, and after separated from the transmission signal therein, is supplied to a receiving mixer 21. In the receiving mixer, the reception signal and the local signal are added to or subtracted from each other thereby to generate an intermediate frequency signal of a frequency f.sub.IF =.vertline.f.sub.R .+-.f.sub.L .vertline.. A demodulator 22 frequency-demodulates the intermediate frequency signal directly or after converting it into a second intermediate frequency signal, and the signal thus demodulated is sent out by way of the terminal 24.
FIG. 2 is a detailed block diagram of the frequency synthesizer 17 and the frequency modulator 18 used in the apparatus of FIG. 1 according to the prior art. The frequency synthesizer 17 includes a reference oscillator 1 for generating a reference signal of a reference frequency, a reference frequency divider 2 for dividing the frequency of the reference signal, a phase comparator 3, a loop filter 4 for smoothing the output of the comparator 3, a local oscillator (voltage-controlled oscillator (hereinafter referred to as "VCO")) 5 responsive to the output of the filter 4, a 2-modulus prescaler 6 for dividing the frequency of the output of the oscillator 5, and a variable ratio frequency divider 7 for dividing the frequency of the output of the prescaler 6. The variable ratio frequency divider 7 is comprised of a programmable counter 8 and a swallow counter 9. The prescaler 6 and the variable ratio frequency divider 7 operate on the principle described in "Frequency Synthesizers Theory and Design" by V. Manassewitch, John Wiley & Sons, pp. 355 to 361, 1976. According to this article, assuming that the two frequency division coefficients or frequency division modulus of the prescaler 6 are, for example, P and P+1, the frequency division coefficient of the program counter 8 is M, and that of the swallow counter 9 is S, the overall frequency division coefficient of a combination of the prescaler 6 and the frequency divider 7 is given by equation (1) below. EQU N=P.multidot.M+S (1)
The phase comparator 3 compares the phase of the output of the reference frequency divider 2 with that of the output of the variable ratio frequency divider 7. The frequency modulator 18 includes the reference oscillator 1 (shared with the frequency synthesizer 17), a reference frequency divider 27 for dividing the frequency of the reference signal, a phase comparator 11, a loop filter 12 for smoothing the output of the comparator 11, a transmitting oscillator (a transmission VCO) 13 responsive to the output of the filter 12, a transmitting mixer 29 for mixing the output f.sub.L of the local oscillator 5 and the output f.sub.T of the transmission VCO 13, a filter 30 for extracting a signal of the desired frequency (frequency f.sub.TIF) selectively from the output of the mixer, and a fixed ratio frequency divider 28 for dividing the frequency of the output of the filter 30.
A configuration similar to FIG. 2 is described in a data book of Motorola Inc. entitled "CMOS/NMOS SPECIAL FUNCTIONS DATA", pp. 6-29 to 6-34, especially, FIG. 4 on p. 6-33, 1984.
Such a known conventional apparatus comprises an analog circuit including the transmitting mixer 29 and the filter 30 in the feedback path from the transmission VCO 13 to the phase comparator 11. In the case of a transmission frequency of 200 MHz to 300 MHz or higher, it is difficult to implement this analog circuit in the form of a monolithic IC. It is, therefore, constructed of a hybrid IC using discrete parts. Also, the local signal (frequency f.sub.L) disadvantageously leaks out to the terminal 15 as an unwanted signal through the transmitting mixer 29. In a similar manner, the transmission signal (frequency f.sub.T) is produced from the terminal 10 as an unwanted signal and disadvantageously leaks out to the receiving mixer 21 (FIG. 1), thereby inconveniently increasing the spurious response of the radio communication apparatus.